1. Field of the Invention
This invention relates to quarter wave plates. It particularly relates to quarter wave plates for use at radio frequencies.
2. Description of the Related Art
As is known to those skilled in the art, a quarter wave plate is a component which produces a phase shift of π/2 radians, i.e. one quarter wavelength (or an odd integer multiple thereof) between orthogonal components of electromagnetic radiation.
Applications of such quarter wave plates include the conversion of unpolarized radiation to circularly-polarized radiation and conversion of plane-polarized radiation to helically-polarized radiation.
It is known to construct a quarter wave plate for use at radio frequencies by using a dielectric. material having an anisotropic relative dielectric constant. Two parallel faces are made on a piece of the anisotropic material. The distance between the faces is such that, in traversing the thickness of the plate, for radiation at the nominal frequency at which the plate is to be used, components in the direction parallel to the axis of the greater dielectric constant undergo a phase shift of one quarter wavelength relative to components in an orthogonal axis having the lesser dielectric constant. One type of material having the necessary anisotropic properties is sapphire. While such plates have been found to produce the necessary phase shift, they suffer a number of disadvantages. Sapphire is relatively “hard” material, i.e., it has a relatively high dielectric constant relative to air. This results in losses by reflection due to the mis-match between free space and the relatively high dielectric constant sapphire. The problem of this mis-match has been addressed by providing anti-reflecting coatings in the conventional manner. While this approach has generally proved satisfactory, problems have arisen from poor adhesion of the coatings to the sapphire. The resulting structure has also been found to have a relatively narrow bandwidth.